When you activate it, you put it in your hand, which makes sense given that it can spawn monsters so putting it in your hand prevents them from spawning I guess. But then it says you can discard it if an attack misses your defending marine. What's the point of this?

The location card that has the artefact doesn't seem to tell you to do anything with it either, just that on entering you have to put it somewhere you want.

Edit: Looks like it's just the wording being ambiguous. I read it as you may discard it IF the attack misses, whereas it's supposed to mean you may discard it to make the attack miss.

Looks like you caught this in your edit, but just to clarify: The artefact card can be used after a defending marine rolls (assuming he rolled lower than the number of genestealers) to make the genestealer attack "miss" rather than slay the marine.